This invention relates to novel coating compositions and more specifically to corrosion-resisting coatings which can be applied directly to various surfaces particularly metal either as a high or low gloss, self-priming topcoat.
Various surfaces and particularly metal substrates require the protection of a coating especially when the surfaces are exposed to a corrosive environment. Metal surfaces of aircraft, for example, are exposed to seawater spray which require protection from corrosion. Specifically, aircraft, e.g., Navy aircraft, are exposed to seawater spray in addition to various acid-forming gases such as sulfur dioxide, carbon dioxide, etc. Moreover, in addition to aircraft, various machinery and equipment in the industrial environments, where fossil fuels are used, need protection against corrosion. It is important therefore that the coatings be resistant to various chemicals and the weather and also be flexible and have good adhesion characteristics.
Presently, coating systems are comprised of one or more films, i.e., an undercoat, a sealant and a topcoat. Aircraft, for example, traditionally have been coated with high performance two-component protective films consisting of an epoxy primer and a polyurethane topcoat. The epoxy primers used on the aircraft are designed to adhere to the metal surface and improve the adhesion of the topcoat and prevent corrosion. However, the primer coat requires a topcoat, since it lacks flexibility especially at low temperatures (-60.degree. F.) which results in extensive cracking in highly flexed areas of the aircraft. The primer also lacks weather resistance and cannot generally be formulated in various colors required for aircraft. Thus, the polyurethane compositions cf this invention provides not only adhesion and corrosion resistance, but also have the required degree of flexibility, the desired optical properties, and the resistance to weather and operational chemicals. Moreover, the multi-film coatings utilized heretofore generally have a total dry film thickness ranging up to about 0.005 inches, e.g., up to about 5 mils or more which adds considerable weight to the aircraft. Further, it is time consuming to apply two coats particularly since there is a drying time requirement between each application. The removal of a two-coat system also can be difficult and time consuming and these coatings generate high levels of volatile organic (VOC) emissions during the coating operations.
In accordance with this invention, however, the corrosion resistant coating comprises a polyurethane binder derived from the reaction of at least one polyol and an aliphatic polyisocyanate based on hexamethylene diisocyanate (HDI) in combination with a unique combination cf corrosion-inhibiting pigments consisting essentially of ar alkaline earth metal or zinc phosphate particularly a zinc-barium phosphate, zinc salts of benzoic acids or substituted benzoic acids, and calcium borosilicate. All three of these salts are essential, in the stated relative proportions, to provide a high gloss film with the necessary corrosion resistance, and adhesion required of a good coating. Other pigments and particularly titanium dioxide (TiO.sub.2) including spherical TiO.sub.2 particles e.g. vesiculated beads may be included as a pigment together with the three salts. The coating composition of this invention may be applied, as one coat, directly to various hard surfaces such as metal and/or organic matrix composites etc. and does not require a topcoat or undercoat to provide a corrosion-resistant finish with desired optical properties.